Referring especially to the problem addressed in inventing the invention herein, vinylidene chloride copolymers (VDC) are preferred for their barrier properties to transmission of moisture vapor and oxygen. The extrusion of VDC copolymers to form tubular films has conventionally been practiced with annular dies having crosshead type mandrels. Spiral type mandrels are not generally used with VDC copolymers because the dwell time is desirably minimized when extruding VDC, and low shear areas are desirably eliminated, to prevent, or delay, the degradation of the VDC copolymers.
It is commonly known that the extrusion of VDC copolymers is accompanied by a plurality of problems relating to the sensitivity of VDC copolymer to the combination of high temperature, and time for which the copolymer must be maintained at that elevated temperature, in the extrusion process.
One problem is that the VDC copolymer sometimes gets into the fit area above the conventionally used crosshead groove, degrades, and washes back out as carbon. The wash out typically is most evident along the weld line of the extruded film. A second problem is that carbon may also form in all areas where the VDC copolymer contacts the metal in the die. With respect to this second problem, the build-up of carbon eventually becomes generally thick enough over a large enough portion of the die surfaces to affect the distribution of the VDC copolymer around the circumference of the die. In addition, bits of the formed carbon may wash out of the die in the extrudate, contaminating the film being fabricated.
These and other sources of problems with the extrusion of the VDC copolymer indicate, and conventional practice verifies, that commercial operation of the VDC copolymer extrusion process can function for only a limited time, such as a few days, between shutdowns for cleaning of the die. Further, this sensitivity of VDC copolymer to the time during which it is held at elevated temperature has resulted in the development of only limited-size commercial dies for fabrication of tubular films, with typical die sizes being less than 25 cm. in circumference. Of course, the larger the die circumference, the greater the amount of time that the polymer spends at the elevated temperature as it travels around the circumference of the crosshead mandrel in the process of being fabricated into a film. Thus, processes for fabrication of films greater than 25 cm. in diameter, and containing the VDC copolymer, are generally carried out in a slot die of the cast extrusion type; and such processes are not available in tubular form, or in a form derived from a tubularly extruded film. While recent developments indicate that die circumferences up to 79.8 cm. have been developed, as in U.S. Pat. No. 4,379,117, Baird et al, such dies at present have limited applicability and are not in wide-spread use.
With the circumference of the die opening thus being limited to no more than 80 cm. in known use, and it is usually no more than 40 cm., the typical blown up diameter of a tubularly extruded film will generally not exceed twice that size, namely about 150-160 cm., usually no more than about 70-75 cm. Those dimensions related to unoriented films as they are fabricated in blown film processes or tubular water quench processes. Where the tube is subsequently molecularly oriented, the tube diameter may be as great as about 300-320 cm., but is usually no more than about 150-160 cm.
It is an object of this invention to provide a method for fabricating tubular multiple layer films containing a layer of VDC copolymer which overcomes the above problems.
It is further an object to overcome especially the problem of the VDC copolymer getting into the fit area above the crosshead groove.
It is still another object to overcome, in large part, the formation of carbon from VDC copolymer in the die.
Still another object is to reduce the fraction of the internal surface area of the die which is exposed to the VDC copolymer.
It is another object of the invention to provide apparatus compatible with the novel methods.
It is another object to provide novel films made by the methods and apparatus of the invention, and packages made with those films.